1. Field of the Invention
The present invention relates to a liquid crystal display apparatus in which alignment division is utilized in a vertical alignment mode to provide a large viewing angle, and a method for producing such a liquid crystal display apparatus.
2. Description of the Related Art
Recently, as office automation equipment, such as personal computers, is becoming more portable, there is an increasing demand for flat-panel display apparatuses. An effort has been made to reduce the costs of such display apparatuses. Examples of flat-panel display apparatuses include a liquid crystal display apparatus (hereinafter also referred to as an LCD), an electroluminescent display apparatus, a plasma display apparatus, and an electrochromic display apparatus. In a LCD, a transmission medium (i.e., liquid crystal) is sandwiched between a pair of substrates on which electrodes are provided. Display is performed by applying a voltage across the substrates to control the electro-optical characteristics of the medium. LCDs are capable of displaying with low power consumption and therefore, has been more widely used.
Display modes and driving methods of LCDs will be described below. Passive matrix LCDs, which utilize super twisted nematic (STN) liquid crystal, can be produced at low cost. However, for passive matrix LCDs, it is difficult to achieve high resolution, high contrast, multiple tones (multi-color or full color) and a large viewing angle.
To overcome the drawbacks of passive matrix LCDS, active matrix LCDs have been proposed in which a switching element (active element) is provided for each pixel so that the number of scanning electrodes can be increased. Such LCDs are on the way to higher resolution, higher contrast, more multiple tones and a larger viewing angle. In active matrix LCDs, each of pixel electrodes arranged in a matrix is electrically connected via an active element to a scanning line passing in the vicinity of the pixel. Examples of the active element includes a two-terminal nonlinear element and a three-terminal nonlinear element. A thin film transistor (TFT), which is a three-terminal element, is a representative active element which is currently used in active matrix LCDs.
A configuration of a conventional LCD will be described below. For example, a voltage-controlled birefringence LCD comprising nematic liquid crystals having negative dielectric anisotropy and a vertical alignment film (hereinafter referred to as a “vertical alignment LCD”) will be described. In LCDs of this type, the transmittance of liquid crystals is controlled by utilizing a difference in refractive index (birefringence) between the major and minor axes of a liquid crystal molecule.
In vertical alignment LCDs, a vertical alignment film is used to cause liquid crystal molecules to be arranged so that the major axis direction of each liquid crystal molecule is oriented in a direction substantially perpendicular to a substrate surface in the absence of an applied voltage across the electrodes provided on opposite sides of a liquid crystal layer. Therefore, incident polarized light, which has penetrated through one of a pair of polarizers orthogonally arranged, passes through the liquid crystal layer without elliptical polarization due to birefringence and therefore cannot penetrate through the other polarizer. In this case, LCDs are in the state of black display. In the presence of an applied voltage across the electrodes provided on opposite sides of the liquid crystal layer, the major axis of the liquid crystal molecule is tilted toward the substrate surface in response to an applied voltage. Therefore, incident polarized light, which has penetrated through one of a pair of polarizers orthogonal arranged, is elliptically polarized due to birefringence in the liquid crystal layer. If a phase velocity difference (retardation) between an ordinary light component and an extraordinary light component in liquid crystals is controlled by adjusting the strength of an electric field in the liquid crystal layer, the transmittance of light outgoing from the other polarizer can be arbitrarily changed. In this case, as an applied voltage is increased from zero, display is changed from black to white.
FIG. 13 is a cross-sectional view showing a schematic configuration of a conventional LCD. This LCD comprises an element substrate 54 and a counter substrate 53. In the substrate 54, three-terminal nonlinear elements 42, such as TFT, and pixel electrodes 44 made of ITO (Indium Tin Oxide) or the like connected to the drains of the elements 42 are provided on a glass substrate 41. In the counter substrate 53, a color filter 47 and counter electrodes 48 made of ITO or the like are provided on a glass substrate 46. Each of the substrates 53 and 54 further comprises a vertical alignment film 49 provided on an inner surface thereof for aligning liquid crystal molecules. A liquid crystal layer 50 having negative dielectric anisotropy is sandwiched between the vertical alignment films 49. Further, polarizers 51 and 52 are provided on an outer surface of the substrates 53 and 54, respectively.
FIGS. 14A to 14C are cross-sectional views for explaining an alignment of liquid crystal molecules in the conventional LCD. As shown in FIG. 14A, in this LCD, liquid crystal molecules 55 in the liquid crystal layer 50 are oriented in a direction substantially perpendicular to the substrates 53 and 54 in the absence of an applied voltage. The liquid crystal molecules 55 are oriented in a direction substantially parallel to the substrates 53 and 54 in the presence of an applied voltage having a predetermined value. As shown in FIG. 14B, if a voltage having a value less than the predetermined value is applied across the liquid crystal layer 50, the liquid crystal molecules 55 are oriented in a direction tilted with respect to the substrates 53 and 54. Thus, rotation or birefringence of light in the liquid crystal layer 50 can be controlled by changing an applied voltage, whereby the transmittance of the light can be arbitrarily changed, i.e., an image can be created. In other words, the orientations of liquid crystal molecules are changed to control retardation, thereby regulating the intensity of transmitted light. In this case, as shown in FIG. 14C, electric lines of force 56 substantially perpendicular to the substrates 53 and 54 are generated between the substrates 53 and 54.
In LCDs of this type, conventionally, protrusions may be provided on the substrate 53 or 54 so as to regulate the orientations of liquid crystal molecules (such protrusions are called domain regulating means).
For example, Japanese Laid-Open Publication No. 2-191914 discloses a liquid crystal electro-optical device as shown in FIGS. 15A to 17B. FIGS. 15A to 17B are diagrams for explaining the orientations of liquid crystal molecules in the presence of an applied voltage. FIG. 16 is a plan view showing a configuration of elements on a substrate of a general LCD. Referring to FIG. 15A, this liquid crystal electro-optical device comprises a counter substrate 113, an element substrate 114, and a liquid crystal layer 110. The element substrate 114 comprises a glass substrate 101, tilted protrusions 103, pixel electrodes 104, and an overcoat layer 105. The protrusions 103 are tilted in substantially the same direction (rightward in the figures). The pixel electrodes 104 are provided on the protrusions 103, so that the pixel electrodes 104 are also tilted in substantially the same direction. Therefore, as shown in FIG. 15C, all electric lines of force 116 in the vicinity of the element substrate 114 have the same tilted direction. With this structure, as shown in FIG. 15B, when a voltage less than a predetermined voltage, which causes liquid crystal molecules 115 in the liquid crystal layer 110 to be substantially parallel to the element substrate 114, is applied between the pixel electrodes 104 and counter electrodes (not shown) on the counter substrate 113, the liquid crystal molecule 115 can be oriented in substantially the same direction. A surface of the element substrate 114 can be made flat with the overcoat layer 105 provided on the pixel electrodes 104. In this case, however, since the liquid crystal molecules 115 are tilted in substantially the same direction, retardation is changed in a relative manner depending on a viewing angle at which a viewer sees a display screen. In other words, the intensity of transmitted light or hue varies depending on a viewing angle, i.e., a so-called viewing angle dependence problem remains.
Further, in the liquid crystal electro-optical device, as shown in FIG. 16, bus lines (electrode line), such as source lines 117 and gate lines 118, are provided in the vicinity of the pixel electrodes 104. An electric field is generated between the bus lines 117, 118 and the pixel electrodes 104. As a result, as shown in FIG. 17A which is a cross-sectional view of FIG. 16 taken along line G–G′, not all the electric lines of force 116 are tilted in the single predetermined direction. Therefore, it is easily inferred that as shown in FIG. 17B, the liquid crystal molecules 115 on a surface of the pixel electrodes 104 are not uniformly oriented, whereby irregular orientation occurs especially in the vicinity of the ends of the pixel electrodes 104.
Alternatively, protrusions 67 as shown in FIG. 18A may be provided on an element substrate 64 comprising electrodes. A vertical alignment film is applied on the protrusions 67. Liquid crystal molecules 65 in a liquid crystal layer 60 are tilted in predetermined directions by utilizing tilted surfaces of the protrusions 67 on the element substrate 64, thereby regulating the orientations of the liquid crystal molecules 65. In this case, however, a distance between the element substrate 64 and a counter substrate 63, i.e., a thickness of a portion of the liquid crystal layer 60 between pixel electrodes, is not uniform.
Japanese Laid-Open Publication No. 11-242225 and Japanese Laid-Open Publication No. 7-199193 disclose another type of LCD. As shown in FIG. 18B, protrusions 77 and 78 are provided on substrates 74 and 73, respectively, in a staggered manner. Liquid crystal molecules 75 on a surface of the protrusions 77 provided on the substrate 74 and on a surface of the protrusions 78 provided on the substrate 73 are tilted in predetermined directions by utilizing tilted surfaces of the protrusions 77 and 78, thereby strictly regulating the orientations of the liquid crystal molecules 75 in the liquid crystal layer 70. In this case, however, the substrates 73 and 74 have to be attached to each other with high precision in order to arrange the protrusions 77 and 78 on the substrates 73 and 74 in staggered and equally-spaced manners.
Japanese Laid-Open Publication No. 6-194656 discloses an LCD of a still another type, in which as shown in FIG. 18C, grooves 89 and 90 are respectively provided in a staggered manner on pixel electrodes 84 and counter electrodes 88 on respective substrates 94 and 93. Electric lines of force 86 are bent in the vicinity of the grooves 89 and 90 on surfaces of the substrates 94 and 93, thereby regulating the orientations of liquid crystal molecules 85 in a liquid crystal layer 80. Also in this case, however, the distance between the substrates 93 and 94 sandwiching liquid crystal is not uniform and the substrates 93 and 94 have to be attached with high precision.
As described above, in LCDs, rotation or birefringence of light in a liquid crystal layer can be controlled by changing an applied voltage, whereby the transmittance of the light can be arbitrarily changed, i.e., an intended image can be created. In other words, the orientations of liquid crystal molecules are changed to control retardation, thereby making it possible to regulate the intensity of transmitted light.
The retardation varies depending on an angle between the major axis of a liquid crystal molecule and the direction of an electric field. Therefore, as disclosed in Japanese Laid-Open Publication No. 2-191914, even if the angle between the major axis of a liquid crystal molecule and the direction of an electric field is controlled in one-dimensional manner by regulating the intensity of the electric field, retardation varies in a relative manner depending on a viewing angle at which a viewer sees a display screen, so that the intensity or hue of transmitted light is also changed, i.e., a so-called viewing angle dependence problem arises. In this case, liquid crystal molecules on pixel electrodes are not uniformly oriented in a predetermined direction, so that irregular orientation occurs especially in the vicinity of the ends of the pixel electrodes.
According to the techniques shown in FIGS. 18A to 18C, the viewing angle dependence problem can be substantially solved. In this case, the distance between substrates sandwiching liquid crystal is not uniform and the substrates have to be attached with high precision.
Further, according to the techniques shown in FIGS. 18A and 18B, an interface (alignment film on electrodes) between a liquid crystal layer and each substrate is in a pit-and-protrusion shape. Therefore, in vertical alignment LCDs, liquid crystal molecules in the vicinity of the alignment film are oriented in directions substantially perpendicular to the pit-and-protrusion shaped alignment film in the absence of an applied voltage. As a result, the liquid crystal molecules are tilted away from directions substantially perpendicular to the substrates by angles corresponding to tilt angles of the pit-and-protrusion shape. Therefore, satisfactory black display is not obtained in a direction substantially perpendicular to the substrates, so that a contrast of a displayed image is significantly reduced.